lnvestigatisn of bonding in tal charge density

I Indian hut. Scr., lilll.-Feh. 1999, 79, 17-30 O Indtan Tnsntut~. of Science

lnvestigatisn of bonding in the solid state using experimental charge density'

G.U. KULKARNI Chcrnlsuy and Physics of Malcnals Unit, Pawahvrlal Nehru Centre Tor Advanced Scientitk Research, Jalakur. Barigalore 560 061.

A med~od or investigating chemcal bonding m crystailme solids using erpenmcmai charge denslty is pescnled. A refinement procedure based on rnult~palc formalism for X-ray diiiract~on data has bcen described along with topog- raphicxl dnaiysis of chvrgc distribution. The method has been applied to n sludy of a and P palymorphs of p- rolrophenol dnd Lhclr photochemicd acllvity. Beades revealing many structural differences, the shldy has brought out signrlicant diffamcea in mlra- md inlermolecular rcsians of the two iotms as seen from deiormtmn density maps. Rel~ef maps of the ncgalive Lapiaciaw m the plane of internnlecular hydmgen bonds show polarization of the oxygen lone-pair elccirms towards hydrogen. Chargc appcars to nugrate from the benzcl~c nng reg1011 orthe molecule to nitro snd hydioxyl groups a? Ihe struckxe tihanps fiom /3 to a form. On prolonged ~rradiation o r the a foim chxge m- grates in the opposite way resembhg more like c h x g distribution in light stable B form. Maleculvr d~polc moments m iulid alate are f o ~ ~ n d lo be cooaidembly largcr thnn the value m the flee mulecale.

Keywurda: Crystal struaure, charge dcnsity, l>-nitrophenol, polymorplusm, photoirradiation

Experimental charge density method using X-diffraction to study chemical bondiug in solid state is becoming increasingly popular amongst chemists in recent years.', "he model is based on a promolecular density obtained as a superposition of the spherical atomic densities centered at nuclear positions. The promolecule can serve as a reference state relative to which charge migration due to bond formation takes place. In order to visualizc charge distribution due to chemical bond formation, a refinement formalism based on nucleus-centered rnultipole expan- sion of the electron density is commonly used.3 Accordingly, aspherical atomic density is described in terns of spherical harmonics.

with the origin at the atomic nucleus. The population coelficients, Ph,, a e refined along with K and K' panmeters which control the radial depcndence of the valence shell. Quantitative analysis of the electronic structure may be carried out by using critical point (CP) search method developed by ~ader." The set of CPs in p(r) at r, is defined such that Vp(r,) = 0. The

*Text of lecture deiwered at the ANIU~ Faculty Meeong of the Jawaharlal N e b Centre for Advanced Scientific Re- search at Bangalore on November 13, 1998

value of p, in a bond measures its strength; the trace of diagonalised Hessian at r, (the Lapla- cian), VZp,, measures the extent of depletion and contraction of charges, and ellipticity, E, obtained as ratios of the Hessian eigenvalues perpendicular to the bond minus one, measures the degree of planarity or conjugation. The position of CP in a bond gives an idea of polarity of the bond. CP tends to shift closer to the more electropositive atom and therefore the polarity of the bond, V, may be expressed as the percentage sWt of CP from mid-point of the bond, normalized with half of the bond length. The vertical displacement, d, of CP from the line joining bonded atoms measures the extent of bending of the bondmg orbitals.

In the last few years, there have been a number of charge density studies on materials of interest in quantum chemistry and medicine. Wozniak et aLS examined attractive inter- and intramolecular N.. .O interactions 1n N, N-dipicrylamine and its ionic complexes. Topological analysis of the charge distribution in maleate salt^^.^ has shown that short hydrogen bonds pos- sess some covalent character. Charge density studies on ~ - d o ~ a ! Vitamin C9 and DL-aspartic acid'' have been reported recently. Some workers have analysed non-centrosymmetric crystals of importance in nonlinear optics.". " Coppens et al.'\arried out charge density studies on long-lived metastable excited states in nitrosyl complexes using synchrotron radiation.

We have initiated a research program recently to investigate charge density in a variety of materials. Our efforts in this direction began with a study of organic crystals in relation to chemical reactivity and polymorphic form?4, l5 We chose p-nitrophenol for this purpose. The a form is known to undergo a topochemically controlled photochemical transformation, manifest- ing itself in an irreversible color change of the crystal from yellow to red. The origin of the color change is however not clear and could be due to a side reaction. The P modification is light stable at room temperature. Thus, p-nitrophenol provides an interesting example of poly- morphism where one polymorph is thermodynamically unstable and the other is photochemi- cally unstable. Previously, some efforts were made to explain this phenomenon based on crystal structure analysis,16. l7 and ENDOR meas~remenis.'~ However, the results are not conclu- sive. We have carried a detailed investigation on P and n polymorphs of p-nitrophenol using charge density method, having collected high-resolution X-ray diffraction data at low tempera- tures on both the forms as well as on inadiated crystal of aform. Our study has shown diffcr- ences in the molecular structure in p and a forms, particularly in the hydrogen bond region and more importantly, significant differences in charge densities and associated properties of the two forms.

2. Experimental

Pale yellow crystals of P and a forms were grown from aqueous and benzene solutions, respectively. High-quality crystals of the two modifications were chosen after detailed examina- tion under optical microscope. X-ray diffraction intensities were measured by w scans using Siemens three-circle diffractometer attached with a CCD area detector and a graphite mono- chromator for MoKa radiation (50kV, 40mA). The crystals were cooled on the diffractometer using a stream of cold nitrogen gas from a vertical nozzle and were held at 1 lOK throughout data collection. Extra care was taken not to expose the a crystal to light for extended hours during the experiment.

EXPERIMENTAL CHARGE DENSITY METHOD

Table 1 Crystal data and experimental details

Chemical hmula C,H,NO, Fornula weight Cell aectmg

Space group a ($1 b ($1 c (A) B ("1 D, (mg m~') Radiation type Wave len~th (A) No of reflections Ibr cell pameters fl (m-0 Ternperamre (K) Crystal h m l

Cryatal size (mm) Cryslal color

Data collection D~ffractomcter Data collection Crysta-detector drstmce (cm) No. of measured reflectims No. of independent reflections No. of observed reflections R,,, R,", ern, ("1 (si118/A),~,),,,, Range of h, k 1

.Refinement (multipole and kappa)

Retinement on F

S No. of reflections used in the refinement No of parameters used Nrni/N" M-abm treatment Weighting schcme

P2,In 3 6812(3) 11.1152(9) 14.6449(12) 92.804(2) 1.544 Mo K a 0.71073 9874 0 13 I10 Needle 0.4 x 0.32 x 0 Pale yellow

Siemens CCD o scan 4.0 9874 4603 4459 0.035 0.035 49.93 1.08 - 7 - i h 1 7 - 2 3 - t k 4 1 6 -28+1+15

Rectangular block .28 0.3 x 0.2 x 0.14

Pale yellow

0.025 0.022 0.032 0.029 1.89 1.80 4459 4045 303 303 14 8 13.4 see iext w = U d ( n = (4~ ' ) ld(P) d ( P ) = dcamm8 (Fa) + P2(f? 0.06 0.01 0.12 0.07

4 . 1 0 4 . 1 1

Irradiated a

P2,lc 6.1414(1) 8.8032(2) 11.5013(2) 103.338(1) 1.527

9880 0.13

Rectangular block 0.3 X 0.23 x 0.15 Red

Table 11 Analgsis at' the bond critical points for the irradiated a (top row), u (middle row) and fi cbottom row) forms

- Bond P

, , 3.3918) 5.0(3) 3.03(2) 10.6(5) 0.6 N 0.03 1

PC (eA3 is the electron density at the eritied point (Vp(r,)=O), V 2 p c (&) the Laplacian (trace of the dngonaL~sed Hessian); efhe ellipticity (rauo of the Hessian eigenvalues perpendicular Lo the bond minus one); A Ule bond polarity (percentage shift of the CP from mid-point of the bond, normalized with half of the bond length); d(A)~le perpendicular distance between the critical point and the internuclear vector.

EXWRLMENTAL CHARGE DENSITY METHOD 21

Initially, unit cell parameters and orientation matrix of the crystal were detemrined n s i ~ ~ g -60 reflections from 25 frames collected over a small w scan of7.S0. A hemisphere of recipro- cal space was then collected in two shells using SMART software'' with 28 settings of the detector at 3 2 O and 70'. The coverage was found to be - 92% using ASTRO routine." Data re- duction was periormcd using SAINT programl"nd crrientalion matrix along with detector and ceil parmeters werc rcfined for every 40 frames on all the measured reflections (Table I). As preliminary check on the X-ray intensity data, crystal structures of the polymorphs were rede- termii~ed using SHELXTZ and were compared with the reported ~tnrctures.'~. l7

As a first step towards charge density analysis, a high-order refinement of the data was carried out using reflections wiih sinO/A 2 0.85 ikL and Fo > 1 0 ~ . The positions of H atom were found using difference Fourier method and were adjusted to theoretical values (C-H, 1.085 A; &H, 0.96 A) and were held constant throughout refinement along with their isotropic temperature factors. As usual, the nonhydrogen atoms were treated anisotropically. These served as initial parameter.; for multipolar refinement.

Multipolar refinement for charge density analysis was canied out using XDLSM routine of the XU package.2' The nonhydrogen atoms were refined up to octapole moments while the hydrogen atoms were restricted to quadrupole moments. Charge neutrality constraint was applied lo the molecule through all cycles of relinernent. r-I'aclors showed si,&ficant improve- mcnt over conventional refinement from around 6% to - 2%. KI-cfine~nent was also cai-ried out foliowing which the residual index showed lwarginal improvement. XDPROP routine was used to calculate total electron density p(r), deformation density Ap and Laplacian v2p (Table 11).

Charge density analysis of the intermolecular hydrogen contacts was canied out using PAKST~' and XD programs (Table 111). Pseudoatomic charges and dipole moments obtained using XDYROP are listed in Table IV.

3. Resmlts and discu$sion

3.1. Structures ofband a forms

In Fig. 1 we show the molecular packing diagrams of Pand aforms, respectively. The molecular structure differs in the two forms to somc extent. We find that OH and NO? groups are not coplanar with benzene rings in both the forms, the constituent atoms of the two groups showing noticeable displacements above and below the mean plane of the benzene ring. In form, N atom remains in the mean plane while 0(3), O(1) and H(1) atoms show small displacements of -0.08, 0.05 and -0.03 A, respectively. O(2) atom of thc nitro group shows the largest displacement of 0.22 A from the benzene mean plane. In a form, on the other hand, H(1) atom oC the OH group is maximally displaced from the benzene ring plane (0.32 A! while N, 0(2) , O(3) and O(l) atoms exhibit small displacements of 0.05,0.04, 0.09 and 0.03 A, respectively.

The C - - C 4 bond angles in p-nitrophenol are not equal (see Fig. I), a feature common to many other phenolic compounds. This inequality is considered to be due to the bent orbitals resulting from repulsion between hydrogen atom and carbon carrying the hydroxyl group. We observe some differences between fi and uforms of p-nitrophenol in relation to this inequality. The value is 6.U0 for P form and 5.5" for afonn. The C-4-N bond angles show small dif-

22 C;. U. KULKAKNI

-- - Bond ~ o n d Hessian egenvalues P (*A ') V X e A '1

length I , 2?b

2 . . 2.382(1) -0 44 -0.27 1.80 0.076(14) 1.147( 10)

0(3)H(13 1.909(1) -0.71 -0.45 5.82 0.11(3) 4.6(,15! 0(1)..-H(3") 2.345(1) -0.25 -0.24 1.14 0.039(13) 0,66(l1 O(2)-H(S7 2.400(1) -0.08 -0.05 0.38 0.007(8) 0.257(41

S-ey codes: a) r-LIZ, -y+l/2+l, z+1/2: b) -x+1/2, y-112, -rcli^ltl: c) -x-1. -y+l, ->+I

Irmdiwted a O(2)--H(1" 2.466(2) -0.10 0.08 0.75 0.03(1) 0.946( 1) O(3)-H(la) 1.881(1) -1.38 -1.04 4.28 0.11(4) 3.07(3! NH(1").466(1) 4 .10 0.08 0.75 0.03(1) 0.9481 1) 0(1).-H(2b) 2.498(1) 4.10 -0.06 1.00 0.039(9) 0.804(2) 0(3PH(3') 2.603(1) -0.12 -0.07 0.72 0.030(4) 0.499(2) 0(2).-~(5~) 2.392(1) -0.21 -0.12 1.21 0.018(7) 0.600(3)

Symmetry codes: a)x+l, -y+ll2, z+1/2; b)-x, )-1/2,-z-lI2 c)-x+l, -y+l,-z; d) -x+Z, -?, -z

ferences. Furthermore, the N--0(3) bond 16 slightly longer by 0.01 A 1i1 a forni, conlparcd to P form.

Table IV Pseudoatomic charges and dipole moments

Atom f l a hidiated a

o(1) 4 . W 9 ) 4.29(10) 4.08(7) o(2) 4.14(9) 4.36(10) -0.10(8) 0(3) 4.2749) 4.30(10) -0.04(9) N 4.47(21) -0.6N22) 4.25(20) c(1) O.OO(13) 0.29(13) 4.06(8) c(2) 4.01(12) 0.32(12) 4.08(7) c(3) 4.M1(13) 0.30(13) 4.15(T c(4) 4.49(13) 0.21(11) O.OO(7) C ( 3 -0.06(12) 0.20(13) 4.13(7) C(6) -0.1M12) 0.30(12) 0.04(7) H(1) 0.46(6) 0.03(11) 0.33(6) H(2) 0.32(7) 0.03(11) 0.11(5) H(3) 0.30(7) -0.02(11) 0.20(5) H(s) 0.30(7) 4.06(11) 0.11(5) H(6) 0.27(8) 0.02(10) 0.12(5) Dipole moment pebye) 21.5 18.0 9.9

EXPERIMENTAL CHARGE DENSITY METHOD

FIG. 1. Moiccularpackmg diagrams of the Pand apolymorphs ofp-nitrophcnol

The crystal packing in P and a modifications is nohccably different. P form crystallizes ig P2dn space group with the cell dimensions of a = 3.6812(3), b = 11.152(9), c = 14.6449(12) A and P = 92.804(2)". We find no orienWional relation of this cell with that of a !om, which occurs in P2,1c space group with a = 6.1664(1), b= 8.8366(3), c = 11.5435(4) A and

p= 103.390(1)". Obviously, the approach of the adjacent inolcculcs is different in thc two structures. The adjacent nonparallel molecules subtend an at& of 74.73" in a fonrr, but they tend to align nearly parallelly in f l form (29.26").

The differences in crystal packing between P and a forms are reflected in the intermulccu- lar hydrogen bonds (sce Fig. 1 and Table In). In 0 structure, there are two nearly parallel H- bonds originating from nitro-oxygens apart from four other bonds involving phenyl hydrogens. In a form, as many as nine hydrogen contacts can be predicted. In both the forms, lhe O(3). . .N(1) contact at -1.9 ,&, with an O(3). . . H ( l H O ( l ) angle greater than 1 60' appears to be the strongest. It is interesting to note that H(l) interacts with the entire nitro group in o: structure and it is possible that the stability of a structure is d a t e d lo such Cavorablc hydrogcn bonding in the intemolccular region. Charge density analysis throws light on this aspcct as will be seen later.

3.2. Charge density analysis

We shall first examine intramolecular bonding in the two modifications of p-nitrophcnol. Thc residual demity maps, obtained as the difference between calculated and exper$cntal densities, were featureless, the magnitude of the random peaks being less than 0.12 e ~ . ' . The slatic deformation density, Ap, obtained as the difference hctwccn total and spherical dcnsiliea with- out thermal smearing, and the map$ are shown in Wg. 2 in the plane of the bznzcx ring. In both p and a: polymorphs, the deformation density builds up as concentric contours in thc regions of C--C bonds of the benzene ring, C-N bond, N 4 bonds of the nitro group as well as in C 4 and O--H bonds of the hydroxyl group. The lone-pair electrons of the oxygeu 2 -

oms are also clearly seen in Fig. 2. In order lo be able to quautilalively compare the iwo charge densities in the polymorphs, we have camed out CP search along different bonds. Regal-dless of the crystal form, the intramolecular CPs are of (3,-1) type with negative Laplacians, charac-

FIG. 2. Static deformation density in the plane of the henzene ring for P and a foms of p-nitrophenol. Contour intervals at 0.1 &".

EXPERIMENTAL CHARGE DENSITY METHOD 25

teristic of a covalently bonded molecular system. In Table 111, we list the bond critical point$ of f l and a fo~ms obtained from thc present analysis.

The electron densities at the-CPs, pep, of the six C-C bonds in the benzene ring vary in a rather narrow range, 2.2-2.35 eA-' ior f l form and 2.01-2.20e.k3 ffor aform, with mean values of 2.26 and 2.09 e ~ . ~ , respectively. The mean values of the Laplacians, VZpCp, ax -22.83 and -18.28 e ~ ~ ~ . Following Cremer and ~ r a k a , ' ~ pcP and VZPCp values correspond to mean bond orders of 2.02 and 1.70 in P and a forms, respectively, akin to thosc in aromatic rings. The ellipticity, &, in both the forms is somewhat lower than the theoretical value of 0.33. These homonuclear bonds are polarized a little, some up to 12% due to perturbation caused by thc two functional groups. The mean dcnsities in the C-H bond regions of tbe benzene rings of the two fonns are similar (-1.9 e k 3 ) and are close lo the theoretical density of 1.846 e k 3 ; the Laplacians are -18.8 and -19.8 eA4 for f l and a forms, respectively. The polarity of C-H bonds, A, is expected lo be 26.6% towards hydrogen and we obtain mean A values close to 21 % in the two forms.

The C(1)--0(1) bond connecting the hydroxyl group to the benzene ring is associated with PCP values of 2.32 and 2.09 e k 3 for f l and a forms, respectively. The corresponding bond orders are 1.43 and 1.23. The bond is polarized towards C(l), with A = 35.4% for P modification (theoretical value -35%) and a lower value of 22.9% for amoditication. The O(1 j H ( 1 ) bond region also exhibits some differences. In this case, pcP value in f l form is lower than that in ol form by 0.23 e k 3 . Fterestingly, however, CPs in the C ( l W ( l t H ( 1 ) region deviate significantly (d - 0.03 A) from the intermolecular vectors due to bending of orbitals (see Table It), which is also reflccted in the incquality of C 4 4 bond angles.

The C(4)--N bond linking the nitro group to the benzene ring is associated with pcp values o i I .91 and 1.78 e k 3 in P and a fonns, respectively, and with the corresponding bond orders of I .03 and 0.93. The bonding within the nitro group varies from one polymorph to the other and also between the two N - 4 bonds of the nitro group. The P form exhibits pcp values of 3.14 and 3.03 eA4 for the N--0(2) and N 4 ( 3 ) bonds, respectively, while the a form shows much higher densities of 3.49 and 3.39 e k l for the same bonds, the latter bcing close to that expected for a double bond. The ellipticity of N 4 ( 3 ) bonds in hoth the fonns is somewhat larger than that of N--0(2) bonds. The Laplacians and the bond polarities of N - 4 bonds are low in both the fonns, possibly because of the presence of inte~molecular hydrogen bonding.

From thc above comparison of charge densities of f l and a forms, it appears that density migrates outwardly from the benzene ring of the molecule to nitro and hydroxyl groups when the crystal structure changes from P to a . Accordingly, the group charges of NO1 are -0.83 and -1.26 in P and aforms, respectively, and of OH are 0.37 and 4 .26 , respectively. In Table TV, we list the pseudoatom charges in the two forms. The calculated molecular dipole moments in the two forms differ to some extent (Table N), but hoth the values are considerably larger than in the free molecule. The experimental value of dipole moment of the free moleculez4 as well as that calculated by usZ5 is around 5.5 Debye. This enhancement of the dipole moment clearly arises from the charge densities in the condensed state and is in line with the observation of Howard et al. for 2-methyl 4-nitroa11iline.'~

The lone-pair electrons on the oxygen atoms of the nitro group occur as (3, -3) critical points in deformation densityl.%nd show significant differences between the two forms. The mean density of the lone pairs is -5.5 c.k3 in /3 form, much smaller than that in a form (-7.6 ek3).' We also notice that the lone-pair lobes are slightly closer to the nucleus in a Form (-0.29 A) as compared to /3 form (-0.39 A). Our later discussion of the intermolecular hydrogen bonding will bring this issuc in right perspective.

Hydrogen bonding between one of the oxygens of the nilro group and the hydroxyl hydrogen of the adjacent molecule (O(3) ... H(1)) is the shortest internlolecular contact (-1.9 A) in both the forms. Charge dcnsity analysis reveals the presence of bond critical points with small densities, 0.11 and 0.19 e k 3 for /3 and a forms. respectively. The Laplacians at the CPs are also small and positive. These observations imply a closed shell interaction in intermolecular hydrogen bonding.%part from the O(3) ... H(1) contact, we have analysed other hydrogen contacts as well. These are weak interactions as seen from the pep and v ' ~ values listcd io Ta- ble In. The two polymorphs exhibit some differences in weak hydrogen bonds, the number of weak contacts being five and eight in /3 and aforms, respectivcly. The 0(2).. .N(2) contact in /3 form is ;early parallel to the main hydrogen bond (see Fig. 1) and exhibits n moderate dcnsity (0.076 compared to the othcr weak contacts. An intcresting featurc in a structure is that a common cntical point is present with a density of -0.016 e k " for thc N. . .Ml) and O(2) ... H(1) contacts, providing evidence for the participation oF thc entlrc nitro group in hydrogen bonding.

The variations observed in h e intermolecular hydrogen bonding region involving nitro group are best represented using relief maps of the negative Laplacian? We show such maps in Fig. 3 for both the polymo~yhs in the plane of Ihe oxygen atoms of nitro and hydroxyl groups iuvolvcd in intermolecular bonding. We see from the figure that the oxygen lone-pair lobes are polarized in the direction of the hydrogen atom in bolh the cases. A major difference between the two forms is that the hydroxyl group appems to have rotated and moved closer to nitro group h a form and that hydrogen is now more closer to the plme formed by oxygen atoms as scen iiom the increase in the intensily o l hydrogen peak in the map. The conclusions dcrived

FIG. 3. Relief map of the negative Laplacian in the plane of h e hydrogen bond. Range, 2 5 0 to 250 eA-'.

EXPERIMENTAL CHARGE DENSITY METHOD 27

from the geometrical structure and critical points of the intermolecular charge distribution are thus substantiated using relief maps.

3.3. Effect of irradiation of a form

We have investigated the geometrical structure and the topography of charge distribution of a form after irradiation. Surprisingly, the geometrical structure showed little variitiou following irradiation in sunlight for several days, although the change in color from yellow to deep red was striking. Charge density analysis however revealed important differences. In Fig. 4(a) we show the static deformation density in the plane of benzene ring for the irradiated a form. A comparison of Fig. 2 and 4(a) shows that the deformation density of the irradiated a form differs from the pristine form and resembles closely that of P form. We have analysed this aspect carefully using CP search (see Table 11).

The mean value of C-C bonds of the benzene ring in the irradiated form is 2.13 eA-3 corresponding to a bond order of 1.77. This value may be compared with that obtained before irradiation (1.70) and also that of P form (2.02). The mean density in C-H bond regions of the benzene ring is very similar to those obtained with the two polymolphs (-1.9 ek3) . The C ( l W ( 1 ) bond exhibits bond order of 1.25 slightly higher than that in pristine form. The O(1)-H(1) bond region exhibits noticeable differences. The pep value in a form decreases remarkably upon irradiation from 2.91 to 2.40 e k 3 . The p form exhibits a moderate density of 2.68 e k 3 . The C(4)--N bond linking nitro group to the benzene ring in a form remains essen- tially unchanged on irradiation (bond order -0.94) though bonding within the nitro group varies significantly. The irradiated crystal exhibits moderate densities of 3.15 and 3.18 eA-3 in N--0(2) and N---O(3) bond regions, respectively, as compared to pristine a (3.49 and 3.39 eA-3, respectively) and P forms (3.14 and 3.03 eA-3, respectively). We notice from Table Il that some of the Laplacians in the nitro group are small positive numbers in contrast to the negative values observed in general for covalently bonded systems. This may be due to higher ionicity of the nitro bonds.

Following irradiation, noticeable changes take place in intermolecular hydrogen bonding. The density with the shortest contact (O(3)-H(1)) decreases from 0.19 to 0.11 e k 3 and is similar to that obtained for the same contact in P form. There are some differences in weaker contacts as well (see Table 111). The density at the common critical point of N--H(1) and O(2)-H(l) contacts increases marginally after irradiation. We find that the relief map of the negative Laplacian shown in Fig. 4b resembles that of pristine a form in that the hydroxyl group appears to have rotated and moved closer to nitro group compared to the situation in P form. However, the lone-pair polarization as well as the magnitude of the hydrogen peak is much less, similar to p form.

From Table IV, we obtain the group charges of NOz and OH of the irradiated crystal to be -0.39 and 0.25, respectively. These values are quite different from those found before irradiation (-1.26 and -0.26, respectively) as well as from those of j form (4 .83 and 0.37 respectively). Charge distribution appears to be less polarized after irradiation. As a result, molecular dipole moment is relatively low in the irradiated crystal (9.9 Debye, see Table N).

FIG. 4(a). Smc defomtion density in the plane of the FIG. 4(b) Relief map of the negative Laplacian in the benzene ring for the irradiated a fom. Contour intervals plane of the hydrogen bond. Range, -250 to 250 eA4. at 0.1 k 3 .

The lone-pa3 elections on the oxygen atoms of the nitro group show significant differences among the three cases. The mean total density at lone-pair CPs is -6.97 eA-3 in the irradiated crystal compared to -7.6 e k 3 and -5.5 e . & ~ ~ in pristine a and P forms, respectively. W e also iind that the lone-pair lobes move slightly closer to the nucleus after irradiation by - 0.02 A.

4. conclusions

The main conclusions from the present study are as follows:

(i) The a form exhibits greater number of intermolecular hydrogen bonds compared to P form which accounts for its thermodynamic stability. The presence of a common CP in the intermolecular region provides a favourable hydrogen bonding in a form.

(ii) The a form retains its geometrical structure even after prolonged irradiation, but un- dergoes subtle changes in the topography of molecular charge density resembling somewhat like that found in the light stable /3 form.

(iii) The charge appears to migrate outwardly from benzene ring to nitro and hydroxyl groups when the crystal structure changes from /3 to a. On irradiation, the charge migrates inwardly and the distribution becomes more even across the whole molecule ac- companying photochemical transition. The dipole moment is also much smaller (9.9 Debye).

The author enjoys collaborating with Prof. C. N. R. Rao, F. R. S., and is also grateful to him for encouragement. The presentation is based on some recent publications and the contributions of Dr P. Kumaradhas and Mr Srinivasa Gopalan are duly acknowledged.

EXPERIMENTAL CHARGE DENSlTY METHOD 29

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